Dehydration/Rehydration Cycle

Dehydration/rehydration cycle [ Botany ]
Synonym: Seasonal Rain/Drought Cycle

Dictionary of botanic terminology
index of names

A recurrent seasonal cyclic process of habit expansion and contraction common in several species of cacti and succulents that gain and loss water.

A dehydration/rehydration cycle is a natural recurrent seasonal process of habit expansion and contraction characteristic of several species of cacti and succulents.
Many cacti and succulents expand and contract depending on the amount of water around. During the wet season or in occasion of unpredictably precipitation they quickly absorb and store water in the branches, stems, leaves or roots then this water reservoirs (Up to 90% of the fresh weight of a cactus may be water) slowly come down and plants bodies reduce in dimensions. Similarly other plants reduces their volumes to concentrate bodily saps in order to became more frost resistant during sever winter times.

In particular many species of cacti and succulents are characterized by a cyclic, both daily and seasonal, process of expansion and contraction, allowing for gains and losses of water. This process is realized in two very different ways:

In columnar species (with ribbed or fluted stems capable of an accordion-like expansion or contraction) this process don’t change the plants height (that remain constant) during the expansion/contraction cycles.
( for more details see: contraction)
In geophytes succulent the mechanism is completely different. In fact even though most geophytic cacti puts on a good amount of new growth each year, hardly get any larger, their dimension remain unvaried year after year, as the individual stems seem to grow no larger, because the old growth tends to compact at the base. The new growth produced during the vegetative season contract considerably and retracts sometimes pulling down completely the plant under the soil during the hottest months of summer and coldest months of winter. It should be noted that "when specimens are in this withdrawn state, it becomes almost impossible to find them in their natural state even though their exact locality is known"

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Holdfast roots [ Botany ]
Dictionary of botanic terminology - index of names

Some species of climbing plants develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place.

Climbing plants, like the poison ivy (Toxicodendron radicans), Boston ivy (Parthenocissus tricuspidata), and trumpet creeper (Campsis radicans), develop holdfast roots which help to support the vines on trees, walls, and rocks. By forcing their way into minute pores and crevices, they hold the plant firmly in place. Usually the Holdfast roots die at the end of the first season, but in some species they are perennial. In the tropics some of the large climbing plants have hold-fast roots by which they attach themselves, and long, cord-like roots that extend downward through the air and may lengthen and branch for several years until they strike the soil and become absorbent roots.

Major references and further lectures:
1) E. N. Transeau “General Botany” Discovery Publishing House, 1994